<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>盒子模型</title>
    <style>
        .wai {
            border: 1px solid red;
            width: 200px;
            height: 200px;
        }

        .nei {

            border: 1px solid blue;
            width: 100px;
            height: 100px;

            margin:50px 50px ;
        }

    </style>
</head>
<body>
<div class="wai">
    <div class="nei"></div>
</div>
</body>
</html>